Today I replaced the two electric screwdriver motors on my
robot with a pair of electric window motors, recovered from
a Jeep at a local scrap car place. These are better than
the screwdriver motors because:

They are a mirror image of each other, so suit a
differential drive perfectly. The screwdriver motors had to
be mounted in opposite directions, using up more space

They use a worm gear, run slower than the electric
screwdriver motors, and have higher torque

They have a proper shaft coming out which I was able to
couple with my axles using standard shaft couplers. I was
never able to remove the drill chucks on the other motors,
so had to use the chucks to couple with axles, which
inevitably led to some slippage

They are a lot quieter than the drill motors, presumably
because they are better quality motors and use a worm gear
instead of what I'm guessing is maybe a dozen conventional
gears

The window motors have proper mounting holes, making
mounting onto the robot chassis fairly easy. The
screwdriver motors were an extremely awkward shape, with no
mounting holes or anything, and the way I was finally able
to mount them was neither elegant nor especially robust

Also, previously I had my optical quadrature encoders
mounted directly onto the shafts from the motors, and I was
never happy with this arrangement. I have now coupled the
encoders using timing belts and pulleys, which is much more
satisfactory.

I still have a small about of backlash between the
wheels
and axles. This is due to the fact that the wheels aren't
really ideal for driving from a drive shaft (they were
intended as free-wheeling wheels). If anybody knows where I
can get a pair of wheels of 4-6" diameter with a good solid
coupling to 1/4" shafts (set screw or pin or whatever),
please let me know at neil@octrix.com.